1. Field of the Invention
The present invention relates to an optical information recording and reproducing apparatus for performing recording of information or reproducing of recorded information with respect to an optical information recording medium. In particular, the present invention relates to a coma aberration correcting technique when information is recorded on or reproduced from an information recording medium having a plurality of recording layers.
2. Description of the Related Art
In recent years, the demand for increased capacity of an optical disc has been increasing. In order to improve the recording density of an optical disc, it is effective, for example, to reduce the diameter of a light beam spot by increasing the numerical aperture (NA) of an objective lens. In recent years, a system in which the NA of an object lens is 0.85 has been put to practical use.
Further, a multilayered recording layer of an optical disc is also extremely effective for increasing the capacity, because the capacity is doubled with almost no change in an optical system. However, because the amount of spherical aberration caused due to a difference in substrate thickness is proportional to NA4, the influence of the spherical aberration is remarkably increased as the NA becomes larger. For example, in an optical system in which the wavelength is 405 nm and the NA is 0.85, because a substrate thickness error of several μm exceeds a diffraction limit, even a single-layer disc requires spherical aberration correction.
When a recording layer of an optical disc is multilayered, an interlayer distance of 10 μm or more is necessary, which further increases the amount of spherical aberration correction. As a means for correcting spherical aberration, a light beam expander, which changes a divergent angle or a convergent angle of light entering an objective lens, has been put to practical use.
Further, recently, in order to improve a system margin, providing a mechanism for correcting coma aberration is becoming the mainstream for products including DVDs for recording. A method of tilting an entire optical pickup is conventionally used for correcting coma aberration. However, since the portion to be tilted is large and heavy, spaces for providing a mechanism for driving the tilt and a driving source are necessary, and thus, there is a problem that the apparatus becomes larger.
On the other hand, a method of tilting only an object lens and a method of correcting coma by a liquid crystal have been put to practical use. In the method of tilting only an objective lens, the objective lens is mounted on a tilt actuator. By tilting the objective lens according to the amount of disc tilt, coma aberration is corrected.
In the case of an ordinary objective lens and an infinite optical system, Japanese Patent Application Laid-Open No. 2005-108938 discloses that the amount of coma aberration caused due to disc tilt and the amount of coma aberration caused due to objective lens tilt are almost equal to each other. Therefore, by driving the disc and the objective lens such that the disc and the objective lens are in parallel with each other, coma aberration can be substantially optimally corrected.
A method disclosed in Japanese Patent Application Laid-Open No. 2000-090479 is an exemplary method of correcting coma by a liquid crystal. In Japanese Patent Application Laid-Open No. 2000-090479, a liquid crystal element has a plurality of pattern electrodes which correspond to the shape of distribution of coma aberration caused due to disc tilt. By changing the voltage applied to the liquid crystal element according to the amount of disc tilt, a coma aberration of an opposite sign is generated, to correct coma aberration.
It is expected that, in accordance with the recent trend toward a larger NA of an objective lens, and toward a multilayered optical disc as described above, an optical pickup will be required to have a mechanism for simultaneously correcting both coma aberration and spherical aberration.
When spherical aberration is corrected by creating uncollimated light using a light beam expander as described above, the ratio of the amount of coma aberration caused due to disc tilt to the amount of coma aberration caused due to objective lens tilt varies according to a transmission layer thickness of the disc.
Therefore, when a read/write operation is performed on a multilayer disc under different conditions of the transmission layer thickness, an optimal amount of lens tilt or an optimal voltage to be applied to the liquid crystal element varies depending on the layer, and, if coma aberration is corrected by the same amount of lens tilt or the same voltage to be applied to the liquid crystal element, coma aberration remains in some of the layers, which lowers the imaging performance, and prevents stable recording and reproducing.